According to a general practice, steel products for the use of, for example, automobile axle unit and engine components, are each manufactured in the manner that the product is hot forged, and thereafter is finished by a machining process (or, “machined and finished,” herebelow). A manufacturing process for such a component is disclosed in, for example, “Plastic Processing Technology Series 4: Forging,” The Japan Society for Technology of Plasticity, published by Corona. The manufacturing process is carried out by processing steps representative of forging production processing steps. More specifically, a material is machined and heated and, thereafter, the thus-processed material is shaped or formed by a forging step and, by necessity, the formed material is heat treated.
Recently, it is increasingly demanded that products for the above-described use be improved in fatigue strength for implementation of, for example, compactness and thinning for weight reduction of automobiles using those products.
In Japanese Patent No. 3100492, as a technique for increasing the fatigue strengths of hot forged products, there is disclosed a manufacturing method for a high fatigue strength hot forged product. According to the method, the entirety of a forged product is hardened or quenched after hot forging and, further, the matrix thereof is precipitation hardened by tempering processing.
Further, Japanese Patent No. 2936198 discloses a cooling apparatus operating such that cooling rate nonuniformity in the entirety of a forged product is eliminated, thereby to control the overall cooling rate for the product.
However, according to the method disclosed in Japanese Patent No. 3100492, the component (product) itself is directly cooled after hot forging, such that the hardness of the entirety of the component is increased, and hence the workability of an area not requiring fatigue strength is reduced. More specifically, according to a general practice, a machine structural component for the above-described use is manufactured in the manner that the material is formed by hot forging into substantially the product shape and, thereafter, the entire surface of the hot forged product is machined and finished. As such, in the manufacture of a machine structural component of the above-described type, the machining process and surface abrading are indispensable. However, in the event that the hardness of the entirety of the component is increased, reduction in machinability inevitably poses a significant problem.
In addition, a manufacturing facility for implementing the above-described method requires a heating facility to provide separate quenching for the precipitation hardening treatment. As such, the facility is not preferable even from the viewpoint of energy saving.
Similarly, the technique described in Japanese Patent No. 2936198 controls the cooling rate of the entirety of the workpiece, such that reduction in machinability poses a significant problem.
Under these circumstances, for the facility described in either of Japanese Patent Nos. 3100492 and 2936198, it is difficult to provide a hot forged product excellent in fatigue properties and cold workability. More specifically, it is difficult for the disclosed facility to provide such a hot forged product that has high fatigue strength, which is required from stress occurred in association with, for example, weight reduction and compactness of the forged product, relative to the forged product obtained from the conventional method, and that has high machinability not only for, of course, an area not requiring fatigue strength, but also for other areas when machining is performed after hot forging, thereby making it possible to be easily finished.
Accordingly, it could be advantageous to provide a hot forging facility enabling manufacturing of a hot forged product excellent in fatigue properties and cold workability.